This invention relates to an EL operating power supply circuit.
Electroluminescent (EL) elements or panels are widely used as EL displays and background light for liquid crystal displays.
The EL element is predominantly a capacitance load when considered as a load, and generally requires AC voltage application for operation. The luminance and corresponding surface illuminance of the EL element largely depends on both the voltage and frequency of alternating current applied thereto. In order that the EL element generate more light, the effective or root-mean-square value and/or the frequency of applied voltage is increased.
No particular measure is required when the EL element is operated directly with an ordinary AC power supply, for example, having an effective value of 100 volts and a frequency of 50 or 60 Hz. To derive more light from the EL element, however, it is necessary to increase the frequency and to increase the voltage within the withstand voltage of the EL element. When the EL element is operated directly with an ordinary 100-volt AC power supply, the maximum voltage applied to the EL element has a peak magnitude of 140 volts (a peak-to-peak magnitude of 280 volts). Most EL elements have a rated withstand voltage of approximately 210 volts (a peak-to-peak value of 420 volts). The permissible frequency has a practical upper limit of approximately 400 Hz in view of the lifetime of EL element.
The frequency of voltage applied to an EL element may be increased by once rectifying an AC power to form a DC power supply of about .+-.140 volts, and alternately opening the positive/negative power supply circuit at the desired frequency f by means of a push-pull circuit, thereby applying AC voltage at the frequency f to the EL element. Generally, power transistors and power MOSFET's are used as the switching elements of the push-pull circuit. An energy loss of such power transistors and power MOSFET's upon switching gives rise to problems that a relatively large heat sink is necessary, a considerable energy loss occurs and the site and manner of locating such transistors must be carefully taken into account.
Therefore, a primary object of the invention is to provide a power supply circuit for operating an EL element which essentially eliminates energy loss, recovers surplus electric charge in the EL element as reverse current flow to the power supply side, and thus increases energy efficiency.